While I cannot speak for the specifics of the particular knock-off(s) being used in this thread--the primary issue I've seen with folks who have trouble with the Ops-Core-style dial liner is improper adjustment for their head of the rear straps/nape.
I've brought this up multiple times before, but on an authentic Ops-Core, there's actually a pretty wide range of adjustment in the rear straps to allow the helmet to be fitted to the individual user's head--I'm not talking about strap length, or the dial and fitband itself--but the actual nape can be adjusted for height and position.
Most of the time I find when people complain about the fit and comfort of a FAST, it turns out they've never adjusted the nape to the shape of their own heads--they're often either borrowing someone else's helmet, or simply took the thing out of the box, tightened the straps, and were never able to get comfortable. To be fair, adjusting the nape is kind of a pain, as you need to remove or at least loosen the rear suspension screws, which hold on the fit-band, straps, and ARC rails, and you have to fiddle and fit the helmet to your head, and then try not to move them while you tighten the screws back down. This is not a procedure that most folks, even experienced military helmet wearers are used to, because most modern 4-point suspensions don't have this kind of adjustment--unlike most suspensions that have napes, the Ops-Core nape is padded, but rigid in design, unlike the soft, conforming napes that most folks are used to, which makes placement of the rigid nape much more critical.
While I'm sure there will still be some people who just don't find the FAST comfortable, no matter how much they adjust them--I think most folks that complain about the comfort of their Ops-Cores could solve the issue simply by properly adjusting the helmet.
A maladjusted helmet may be fine during the day, and/or without anything mounted to it--even while poorly adjusted, and especially with a lightweight bump helmet, it simply might not be as noticeable, and the nape itself may "adjust itself" to the wearer's head to conform to its placement. With a NOD mounted to the front, it becomes a different matter entirely--the weight up front pulling the helmet forward and down puts a lot of pressure on the nape for stability--meaning that the weight up front will driving the nape into the wearer's skull in entirely the wrong place for their head, and thereby being exceedingly uncomfortable. Needless to say, while I personally am a fan of Ops-Core helmets--I too have worn a poorly adjusted one, and if it's not adjusted right--it's not fun.
One the one hand, I've never heard of the weight of just a mount suddenly causing issues because of weight, when the wear of the helmet did not--but I suppose it is possible that again, with a lightweight bump, the weight of the mount is enough to shift the helmet ever so slightly, and placing the nape in extremely uncomfortable position--one that can be exacerbated if it's a dial-liner, and the user is cranking down on the dial, "locking it in" in the most uncomfortable place possible.
Either way--my first thought would be to double check the fit and adjustment of the helmet.
~Augee